The present invention relates to high energy density capacitors and metallized films for making high energy density capacitors which have improved self healing properties, that is capacitors which clear a short circuit fault without undergoing a catastrophic failure. More particularly, the present invention relates to improved metallized films having fusible features, whereby a short-circuited part of a capacitor constructed from such a material may be removed from active use. Further, the invention relates to capacitors made from such films.
Standard metallized film capacitors are formed of a material including two thin metal film layers (the plates) separated by a thin sheet of dielectric film, e.g., polyester, polyethylene terephthalate (PET), etc. A capacitor comprises a long strip of such metallized dielectric film material wound into a cylindrical form.
Electrical leads connect the capacitor into a circuit. One lead connects a branch of the circuit to each of the plates. One plate of the capacitor is metallized up to and including one edge of the dielectric film, the contact edge, while an opposite edge of the dielectric film is left clear of metal, the clear edge. The other plate of the capacitor is similarly arranged, but with the contact edge and clear edge reversed. After winding, the exposed contact edges are spray terminated, as is known in this art, to provide a direct electrical contact to all parts of each plate from a corresponding lead.
Such metallized film capacitors have a unique self healing property, for which they are considered useful. When a metallized film capacitor is exposed to an excessive voltage, a short circuit may develop through the dielectric film, between the plates of the capacitor. A substantial short circuit current may temporarily flow through the short circuit. However, the short circuit current flowing through the short circuit region often burns away part of one of the metal layers in the vicinity of the short circuit, opening the short circuit, much like a fuse.
The dimensions of a capacitor, its maximum working voltage, and its capacitance define the energy density of the device, according to: ##EQU1## where C is in farads, E is in volts and V is in cm.sup.3. Although dielectric films currently in use have dielectric strengths of about 500V/.mu.m, a typical conventional capacitor using 6.mu.m dielectric film has a maximum working voltage of about 380V. A conventional capacitor using 8.mu.m dielectric film has a maximum working voltage of about 450-480V. Exceeding these values in a component not designed for such stress levels can result in an unsafe condition in which a high short circuit current discharges substantially all the stored energy in the capacitor through the fault, causing the catastrophic destruction of the capacitor. Such catastrophic destruction occurs because the high current carbonizes some surrounding dielectric, resulting in an avalanche of current discharged through an ever-increasing fault.
In a more advanced conventional capacitor, made for example from the materials shown in FIGS. 1 and 2, segmented metal film capacitors take the fuse concept a step further. In such structures, a plurality of metal regions within the capacitor are interlinked by small fuses. When a short circuit develops in one of the plurality of regions, the fuses interlinking that region to adjacent regions will fail, removing the defective region from the capacitor, avoiding a catastrophic failure of the capacitor or the circuit in which the capacitor is used.
In the plan views of FIGS. 1 and 2, only one of the two metal film layers is shown. However, it should be understood by those skilled in this art that a substantially corresponding pattern may be formed by the second metal film layer, so that the two plates, separated by the dielectric film, overlap for substantially all of their areas. Alternatively, the second metal film layer may overlap substantially all the area of the first metal film layer, but without slits (FIG. 1, 107) or non-metallized areas (FIG. 2, 205), which are explained below, i.e., the second metal film layer may be metallized in areas corresponding to slits (FIG. 1, 107) or non-metallized areas (FIG. 2, 205).
In the conventional segmented material shown in FIG. 1, a polypropylene film 101 carries a metallization pattern 103 in which a plurality of metallized segments 105 separated by slits 107 along most of their length are connected at one edge by small fuse areas 109.
In another conventional material shown in FIG. 2, a polyester film 101 carries a metallization pattern 201 in which a plurality of substantially square regions 203 are separated by surrounding non-metallized areas 205. Small fuse areas 207 may interconnect adjacent square regions 203. This pattern is referred to hereinafter as the "checkerboard" pattern. In this structure, when an individual square 203 (the affected square) develops a short circuit, the fuse areas 207 interconnecting the affected square 203 to adjacent squares 203 break, isolating the affected square 203.
In each of these conventional designs, the total capacitor area may be on the order of several square meters. The segments or square regions are generally on the order of a few square centimeters or less. Thus, the change in capacitance which results from isolating a segment or region in which a short circuit has developed is small, typically much less than 0.01%.
One problem with conventional segmented capacitors, particularly those employing the checkerboard pattern, is poor long-term capacitance stability due to electrochemical erosion of the metal film layers. The effect is pronounced at sharp corners or edges, of which the checkerboard pattern has an abundance. Due to the size of the squares and the number of corners and edges found in a typical capacitor, electrochemical erosion can account for a change in capacitance of up to 5%. By comparison, the loss of only 1 or 2 1cm.sup.2 squares out of 5m.sup.2 of metal film plate area, produces a change in capacitance of only 0.002-0.004%. The segmented design suffers less from electrochemical erosion because it does not have as many sharp corners in the pattern. However, the segmented design of FIG. 1 is less able to withstand excessive voltages, since it is constructed of fewer, larger segments. Moreover, the larger segments require higher fault currents to clear a short circuit fault due to their larger area than the squares of the checkerboard pattern.